A single-wall carbon nanotube (SWNT) is a cylindrical structure formed by rolling up a graphene sheet. The direction and magnitude of the rolling vector define the chirality and diameter, respectively, of the resulting nanotube and determine whether the nanotube is metallic or semiconducting. Common methods used to synthesize SWNT produce complex mixtures that contain many chiralities. Statistically, one third of the mixture is metallic while the remaining mixture is semiconducting. However, many applications of SWNTs, such as conductive films and high-performance field-effect transistors, require enrichment of nanotubes with metallic and semiconducting properties, respectively. Consequently, there have been intense efforts to develop various sorting techniques for separating SWNTs, including selective chemistry, dielectrophoresis, selective oxidation, ultracentrifugation and DNA wrapping chromatography. Several of these methods have been demonstrated to achieve high-purity separation of metallic and semiconducting SWNTs. However, these methods suffer from problems including low yield or high cost.
Carbon nanotubes have been separated by both physical (electrophoresis and centrifugation) and chemical methods (chromatography, selective solubilization and selective reaction). Electrophoresis has been employed for separating various kinds of SWNTs, synthesized by laser vaporization, arc discharge, chemical vapor deposition (CVD) and HiPco (high pressure CO) process, according mainly to electrical property (metallic/semiconducting) together with length and diameter. Charged biological macromolecules are commonly separated by electrophoresis using gel in an electric field. In order to process SWNTs in the gel, however, the SWNTs have to be individually dispersed with aid of surfactant such as sodium dodecylsulfonate (SDS), sodium cholate (SC) and sodium dodecylbenzenesulfonate (SDBS). This type of metallic/semiconducting separation utilizes different polarizable characters between the nanotubes under an electric field. Dielectrophoresis technique was first used for the alignment and purification of SWNT bundles in isopropyl alcohol and has been extended to separation of individually dispersed SWNTs.
Sodium dodecyl sulphate (SDS) and/or sodium cholate (SC) have been commonly used as detergents to dissolve SWNTs. In 2005, Arnold and Hersam disclosed, in Nature Nanotechnology, 1, 60-65 (2006), an example of density gradient ultracentrifugation (DGU) for separation of the diameter of SWNTs. They used structure discriminating surfactants and applied the DGU to separate metallic/semiconducting nanotubes.
Covalent and non-covalent sidewall chemistries to selectively impart ionic character to metallic or semiconducting carbon nanotubes also have been developed. For example, Woo-Jae Kim et al. disclose in Chemistry of Materials, 19, 1571-1576 (2007), that selective covalent sidewall functionalization of metallic SWNTs can be achieved with p-hydroxybenzene diazonium salt after which a negative charge can be induced on the metallic SWNTs through deprotonation in alkaline solutions, thus enabling subsequent separation by electronic type using free solution electrophoresis.
In US Patent Publication No. 2010/0101983A1, Butler et al. describe a flow sorting method of detecting and separating carbon nanotubes based on a electrophoretic method. In particular, the method involves focusing the dispersion of individually suspended carbon nanotubes into a single file stream in a microfluidic device, and detecting and sorting of metallic and semiconducting nanotubes.
Each of these techniques, however, is disadvantaged in that they are not readily scalable, suffer from low yield, or are expensive. There is clearly a need, therefore, for efficiently sorting semiconducting and metallic nanotubes using a scalable, high yield, or low cost technique.